1. Field of the Invention
The invention relates to a test piece for use of X-ray inspection, and more particularly to a test piece to be used when silicide alloy formed in a semiconductor device is inspected with X-ray.
2. Description of the Prior Art
With the increasing integration these days due to down-sizing of a MOS transistor, a gate width is required to be smaller and smaller. Accordingly, when a microstructure having an order below a sub-micron is to be manufactured, a parasitic resistance such as a lead wire resistance and a contact resistance tends to increase. As one of solutions to this problem, "salicide", which is an abbreviation of self-aligned-silicide, draws attention. In salicide, an area in which silicon is in exposure, such as source/drain area and a gate electrode, is reacted with metal which is to be formed on the area, to thereby form a silicide film in self-alignment. The salicide technique is compatible with silicon process having been used so far, and further has many advantages such as decreasing a resistance of a conjunction which has been shallow and thereby caused a higher resistance with a semiconductor element being smaller and smaller, decreasing a contact resistance relative to aluminum, and highly densifying a pattern layout by applying to internal wiring layers. Thus, the salicide technique is indispensable for manufacturing a micro semiconductor device having a size of sub-micron order.
For instance, hereinbelow is explained a process for forming titanium silicide (TiSi.sub.2) by means of a lamp anneal apparatus. First, there is formed openings in oxide film having been formed on a surface of silicon, to thereby expose a surface of silicon to atmosphere. Then, titanium is deposited in the openings. Then, titanium is reacted with silicon under a heat treatment at 700 degrees centigrade for about 30 minutes by means of a lamp anneal apparatus, to thereby form titanium silicide having C49 structure composed of metastable layers and also having a specific resistance in the range of 60 to 70 .mu..OMEGA..multidot.cm. Then, excessive titanium and nitrided titanium are removed using a mixture solution of ammonium and hydrogen peroxide. Then, a resultant is thermally treated at 900 degrees centigrade for about 10 seconds to thereby form titanium silicide having a low resistance, more specifically, a resistance in the range of 15 to 18 .mu..OMEGA..multidot.cm.
On the other hand, with a semiconductor device being down-sized, a diffusion layer is required to have a shallower depth for preventing latitudinal diffusion of impurities. However, if a conjunction interface of silicide is coincident with that of a diffusion layer, an electrical leak might occur at the conjunction interface. In order to prevent such a leak, a thickness of silicide film is required to be thinner.
However, to make a thickness of silicide film to be thinner entails an increase of a resistance of a silicide layer, and further poses two problems with respect to physical properties on the formation of silicide, for instance, physical properties of titanium silicide. One of the problems is a disconnection in a silicide layer due to cohesion, and the other is an increase of a temperature at which C49 structure is phase-transformed to C54 structure.
The reason of the disconnection in a titanium silicide film due to cohesion is considered as follows. The titanium silicide film begins to soften at 800 degrees centigrade, and then is fluidized. The fluidization occurs on a surface of a titanium silicide film or an interface of a titanium silicide film and a diffusion layer. Due to the fluidization of the titanium silicide film, titanium silicide is deformed so that a potential energy thereof is minimized. Thus, the titanium silicide is changed into island areas and thin film areas. Thus, the titanium silicide film loses non-uniformity about a thickness of a film, and further may introduce a complete disconnection therein, Accordingly, an electrical conductivity of the titanium silicide film is deteriorated, and a resistance of the film is increased. In addition, the decrease both in thickness of a silicide film and width of silicide wiring causes an increase of surface tension, and a temperature at which the cohesion begins in a thin film is lowered with a decrease of a thickness of the titanium silicide film. FIG. 1 shows a relationship between a resistance of a silicide layer and a lamp annealing temperature. By reducing a thickness of the silicide from 50 nanometers to 30 nanometers, a temperature at which cohesion begins can be lowered from approximately 900 degrees centigrade to approximately 870 degrees centigrade. Refer to the article authored by T. P. Nolan, R. Sinclair, and R. Beyers in J. Appl. Phys, 71(2), 15, 1992.
The formation of titanium silicide having low-resistivity needs a phase transition from the C49 structure which is a metastable phase and has a high layer resistance (a specific resistance is 2.times.10.sup.-4 .OMEGA..multidot.cm) to the C54 structure which is an equilibrium phase and has a low layer resistance (a specific resistance is 1.5.times.10.sup.-5 .OMEGA..multidot.cm). To manufacture a transistor in microstructure is required to increase a concentration of impurities and also decrease a thickness of a silicide film. A temperature at which a phase is transited varies in dependence on the concentration of impurities and the thickness of a silicide film. FIG. 2 shows a relationship between X-ray intensity of C49 titanium silicide (131) and a lamp annealing temperature. By reducing a thickness of a silicide film from 50 nanometers to 30 nanometers, the phase-transition temperature from C49 to C54 is raised from approximately 850 degrees centigrade to approximately 950 degrees or more. Refer to the article authored by H. Jeon, C. A. Sukow et al. in J. Appl. Phys, 71(9), 1, 1992.
Accordingly, titanium silicide can be formed in a temperature range limited by the phase transition temperature and a cohesion temperature. Thus, in a thin film silicide forming process, the phase transition temperature is an indispensable physical constant for controlling properties of a semiconductor device. There has been known various processes for determining a phase transition temperature of a high fusing point silicide, such as a use of a differential scanning calorimeter (DSC) and a direct observation with an electron microscope.
FIG. 3 is a schematic view of DSC. The DSC uses a thermo-balance placed in an inert gas atmosphere in a constant temperature bath 20. With an ambient temperature being raised at a constant rate by means of a heater 21 composed of Fe--Cr, a difference in temperature between balances on each of which is placed a reference sample 22 and a test sample 23, respectively, is successively measured with a thermo-couple 24. Then, a qualitative analysis is conducted based on the change in temperature of the test sample, and a quantitative analysis is conducted based on an area surrounded by a curve representing a relationship between a temperature difference and time. In this process in which DSC is used, the determination of a phase transition temperature is based on a quantity of heat of the test sample and the phase transition temperature. In other words, when a thin film formed on a silicon substrate merely by approximately 500 angstroms is to be tested for determining a phase transition temperature thereof, a quantity of heat of the thin film lowers sensitivity of measurement. In addition, thermal noise generated with a temperature raise also lowers sensitivity of measurement. For instance the sensitivity is 0.02, 0.2 and 5 mw at 500, 750 and 1000 degrees centigrade, respectively. Thus, the DSC method is not suitable for measuring a phase transition temperature of a high fusing point metal.
On the other hand, an X-ray diffraction process can easily determine a transition of crystal structure. FIG. 4 schematically illustrates a goniometer optical system for use with an apparatus for measuring diffracted X-ray using a X-ray diffraction process. A goniometer optical system for use with a diffracted X-ray measuring apparatus has a main purpose of enhancing diffraction angle resolution to thereby increase accuracy with which a spacing between crystal lattices is to be measured. In a goniometer optical system, an X-ray supplying source 31 transmits X-rays to a test sample 32 with an angle .theta.. Then, a diffracted X-ray present symmetrically about a normal line 37 of the test sample 32, namely a diffracted X-ray present at an angle 2.theta. with respect to the incident X-ray, is detected by a diffracted X-ray detector 33. The incident X-ray and diffracted X-ray are converged on a scanning circle 34 of the X-ray detector 33 (beam centralizing process). The goniometer optical system is designed based on a symmetrical reflection process as aforementioned, which is so-called a ".theta.-2.theta. process". The incident X-ray goes through a incident solar slit 35, and the diffracted X-ray exits through a receiver solar slit 36. The symmetrical reflection process as aforementioned provides high accuracy with which the diffracted X-ray measuring apparatus can measure a spacing between crystal lattices, and also provides crystal orientation.
A rate of an entire diffraction intensity G.sub.x in X-ray diffraction for a surface layer which is present at depth X is denoted by the following equation. EQU G.sub.x =1-exp [-.mu.X(1/sin .gamma.+1/sin .beta.)]
wherein .mu. represents an absorption efficiency, .gamma. represents an angle of incidence, and .beta. represents an angle of reflection.
Assuming that if the diffraction intensity by the above mentioned surface layer is 95% of the diffraction intensity of an entire test sample, the diffraction intensity is a sufficient value, the diffraction intensity by a portion present below the surface layer is disregarded, and then the depth X is an effective depth. Accordingly, a diffraction intensity is exponentially decreased in a thin sample having a thickness thinner than an effective penetration depth X.sub.1 of X-ray. Accordingly, the larger an angle of incidence .gamma. and an angle of reflection .beta. are and also the smaller an absorption efficiency .mu. is, the larger the rate of an entire diffraction intensity is. It is preferable to use an incident X-ray having a large angle 2.theta. and also having a short wavelength.
However, when a chemical structure and a lattice constant of a thin surface layer such as a silicide layer, for instance, a titanium silicide layer is to be inspected, the angle .theta. has to be small in order to obtain an effective layer thickness through which X-ray can diffract. On the contrary, the angle .theta. has to be large in order to accurately measure a lattice constant. Thus, a conventional X-ray diffraction process poses a problem that a thin film cannot be measured with high accuracy.